Numerical modeling of tin-based absorber devices for cost-effective solar photovoltaics.

摘要

Due to the pressures of decreasing the electricity generation costs from solar photovoltaic (PV) modules, there is a need for novel light absorbing materials that can promise comparable conversion efficiencies at lower manufacturing costs than the incumbent technologies based on crystalline Si or thin films (CdTe or Cu-In-Ga-S). This research evaluates the tin-suite of absorber materials (based on tin monosulfide; SnS, and Cu-Zn-Sn-S; CZTS) as the next generation of PV cells that can yield the desired performance in the long term. Numerical models have been developed using Analysis of Microelectronic and Photonic Structures (AMPS-1D) to reveal efficiencies of cells under AM1.5 illumination based on three n-p heterojunctions: CdS|CdTe, CdS|SnS and ZnO|SnS, and identify avenues for further efficiency improvements in SnS PV. It has been predicted that PV devices based on the SnS (absorber)-ZnO ( oxide) configuration yield higher conversion efficiencies (eta=20.3%) compared to the inverted configuration oxide-absorber, with respect to the incidence of light. The difference has been attributed to variations in open-circuit voltages for the two situations, and suggests the adoption of the absorber-oxide design for further device development. The other approach to increasing open-circuit voltages in tin-based cells by using the wider band gap CZTS revealed upto 17.6% efficiency limits in FTO (F-doped SnO2)-oxide (ZnO)-sulfide(CdS)-absorber(CZTS) baseline cells, lower than the absorber-oxide configuration in SnSbased PV. Modeling of the CZTS cells under inverted configuration showed remarkably poor efficiencies (~3%) due to high hole affinities in the absorber that promoted surface recombination at the absorbermetal ohmic contact. This observation suggests the requirement of smaller hole affinities and indicates optimal absorber band gaps of 1.2-1.3 eV for high-efficiency PV conversion. The optimal value should be achieved by cationic substitution of the tin sublattice with copper and/or zinc atoms rather than substituting sulfur with selenium in CZTS. It is concluded that future research efforts in device development should utilize the absorber-oxide design wherein the absorbers have the optimal band gaps as stated. The role of simulation tools such as AMPS will be crucial in aiding critical decision making on tin-based materials as solar PV continues to become an affordable source of electricity.